The invention relates to a method for measuring distance between two objects using electromagnetic waves. The invention also relates to a method for controlling access to an object or a use of an object, in which methods measures are taken to prevent unauthorized use of or access to the object. The invention also relates to an access control and driving authorization device for a motor vehicle.
Access control systems, immobilizer systems or other control systems whose function is enabled or controlled only when there is confirmed authorization of a code emitter 15, for example in the form of a smart card or an electronic key, which is interrogated in a wire-free fashion, are known for motor vehicles (U.S. Pat. No. 5,293,160). Usually, for this purpose at least one base station with a transceiver unit for controlling access or the use of the vehicle is arranged on or in the motor vehicle.
In reaction to a triggering event, for example the activation of a door handle, of an ignition starter switch or the like, an interrogation code signal is emitted at regular or irregular intervals and receives a response in the form of a response code signal from an authorized code emitter which is carried by the user (this is also referred to as an interrogation/response dialog).
The base station checks the received response code signal for its authorization, for example by comparing an item of code information contained in the response code signal with a stored item of reference code information. In the case of authorization, the desired function, such as unlocking of the vehicle door locks or the release of the immobilizer, is carried out.
In such a system, security problems may arise, for example owing to unauthorized interception and manipulation of the wire-free communication between the motor vehicle and code emitter. In addition, code emitters which are too far away could permit access to the motor vehicle. For this reason, in the known access control system the transient time of the signals is measured. This transient time measurement is used to calculate the distance between the code emitter and the base station. Only if the distance is within a predetermined range can the code emitter control the access or the use by means of its response signal.
As the signal transient times are relatively short in free space in comparison with the calculation times which are required at the code emitter to generate the response code signal, a transient time measurement has considerable tolerance problems.
The patent U.S. Pat. No. 4,757,315 has disclosed a method and a device for measuring a distance, in which method and device a station emits a first signal with a frequency f1 and in response a transponder transmits back a second signal, with a frequency f2, which is generated passively from the first signal with the frequency f1. The station receives this signal and determines the distance between the station and transponder from the phase difference between the two signals.
The patent U.S. Pat. No. 5,723,911 discloses a method for keyless access control, which uses a measurement of the distance between a motor vehicle and a transceiver to prevent unauthorized access.